(1) Field of the Invention
The present invention relates to a plastic cladding composition for a plastic core optical fiber, and further, to a plastic core optical fiber prepared from this cladding composition.
(2) Description of the Related Art
A plastic clad plastic core optical fiber prepared by conjugate spinning of a thermoplastic polymer as the core and a thermoplastic polymer as the cladding material has an excellent bending resistance and, therefore, optical fibers of this type have been widely used as light information transmitting media in the field of factory automation and the field of short-to-medium distance communication, such as short distance communication in an automobile, or for a large-scale display device. The application ranges of optical fibers of this type are now being broadened.
However, since the heat resistance temperature of plastic optical fibers used in these fields is about 100.degree. C. at highest, the uses and way of use are restricted and must be improved.
As a means for satisfying this requirement, a method for preparing a heat-resistant plastic optical fiber by conjugate spinning of a heat-resistant polymer such as a polycarbonate or 4-methyl-pentene-1 polymer as the core and a thermoplastic polymer having a low refractive index as the cladding material is proposed in Japanese Unexamined utility Model Publication No. 50-44544 or Japanese Unexamined Patent Publication No. 57-46204, No. 57-179805 or No. 60-32004.
The heat-resistant plastic core optical fiber prepared according to this method appears to have an appropriate heat resistance, but when the fiber is exposed to a high-temperature atmosphere for a long time, the light transmittance thereof is drastically degraded This is because, since this plastic clad plastic core optical fiber is prepared by conjugate spinning conducted at a very high temperature, the plastic material undergoes thermal degradation during the spinning process. Especially, since the heat resistance of the thermoplastic polymer cladding is not high, the thermoplastic polymer is seriously damaged by the high-temperature exposure in the conjugate spinning process.
U.S. Pat. No. 4,511,209 discloses a cladding composition for plastic clad silica core optical fibers, which comprises a highly fluorinated monofunctional acrylate, a polyfunctional acrylate being trifunctional or higher serving as a crosslinking agent, a monofunctional or trifunctional thiol functioning as a synergist, and a photoinitiator. In order to maintain a good pliability in the formed polymer cladding, the highly fluorinated monofunctional acrylate of the cladding composition has an alkyl radical substituted with at least 12 fluorine atoms, and a specific thiol compound such as .gamma.-mercaptopropyltrimethoxysilane is used as the synergist.
According to this known technique, the compatibility of the cladding composition with the crosslinkable polyfunctional acrylate is poor and, therefore, the plastic cladding often becomes turbid and it is difficult to obtain a plastic clad silica core optical fiber having excellent light transmission characteristics.